Noise is produced by all electrical components, as well as by ambient conditions surrounding circuitry (e.g., temperature). Noise can be passed into an electrical system by external components, and may also produced by the electrical system itself. Consequently, the intrinsic noise level of a system determines the lower limit or minimum signal strength of a signal that can be detected in the presence of noise. This is usually defined in receiver terms as the system ‘sensitivity.’
Typically, the first action of a receiver in detecting an incident signal from an antenna is to amplify the signal's magnitude to make subsequent detection and processing stages easier. This is achieved using an amplifier which increases the amplitude of the signal (S) while simultaneously minimizing the amount of additional noise (N) power added to the signal. It is therefore desirable to maximize the signal-to-noise ratio (S/N) of signals in the receiver. Low Noise Amplifiers (LNAs) accomplish this goal of increasing signal (S) power without concurrently increasing noise (N) power. LNAs are typically implemented in stages, such that each stage includes one or more amplifiers.
The reason for implementing LNAs in stages is as follows. The noise contribution of a single amplifier is a function of many fundamental parameters such as the device type, and its operating bias-point. In addition, other considerations such as the source and load impedance presented to the device, have a particularly strong influence on both the added noise contribution from the amplifier to any input signal, as well as the subsequent gain of the input signal. The magnitude of the input signal is increased by the gain of the amplifier by the same magnitude, as is the incident receiver noise. Any additional noise superposed with the received signal therefore has the overall effect of reducing the S/N ratio. It is therefore more preferable to minimize the noise contribution of the first stage amplifier at the expense of gain, and increase signal magnitude in a second or third stage where the effect of added intrinsic noise is less deleterious to the integrity of the received signal. Thus, by implementing a multi-stage amplifier, the effects of noise can be substantially reduced.
The usual technique for reducing noise in a multi-stage amplifier is through careful inspection of the noise circles or contours associated with the first stage of the multi-stage amplifier. Noise circles (contours) are loci on the complex impedance plane of the magnitude of added noise for a given device and operating point. The source impedance point that results in minimum added noise is selected, and an input matching-circuit implemented. A similar output matching circuit is designed to maximize the signal gain of the amplifier for a given load impedance. Additional amplifier stages are then added which have similar input and output matching circuits to further maximize the gain of the overall circuit, while ensuring that other important criteria such as stability are met. The result of not including such appropriate matching circuitry is often reduced amplifier gain, lowered efficiency, and instability.
The above-described design technique requires the use of lumped element capacitors, resistors, or inductors, or at high frequencies, synthesized matching elements (distributed) through the use of transmission lines. In an integrated circuit (IC) implementation, it is often desirable to minimize the real estate occupied by these elements in order to reduce the production cost of the circuit, by both increasing the number of die/wafer, and increasing the yield (functional number of die/wafer).
In addition, in Silicon (Si) IC implementations, the losses generated by the input, output and inter-stage matching circuits may be substantial. At high frequencies (e.g., greater than 5 GHz), Si is a relatively poor semiconductor substrate with high signal loss. It is therefore possible that the increased signal gain achieved through the use of these input, output, and inter-stage matching circuits is of a similar order of magnitude to the loss introduced by these same components. Similarly, in minimizing the added noise of the first stage amplifier, any loss introduced by input matching circuitry may be of a similar magnitude to the noise produced by an unmatched first stage amplifier. It is therefore possible and conceivable that there could be an instance whereby the overall noise and signal gain of a cascaded amplifier that contains input, output, and inter-stage matching circuitry, could be only marginally, or at worst, no better, than a similar unmatched amplifier should the matching elements have high loss.
In this instance it may be preferable to employ an unmatched amplifier (assuming that stability and other operational parameters are met) due to the advantages it possesses in having reduced size.
As discussed above, the overall noise figure of a multi-stage Low Noise Amplifier (LNA) is affected by the input matching network which couples the input signal to the first amplifier stage, and the inter-stage matching networks disposed between amplifier stages. In integrated circuit (IC) implementations, size reduction is important to reduce cost. Thus, the use of on-die matching networks is an attractive possibility. However, at higher frequencies (e.g., millimeter (mm) wave), and on lossy substrates (e.g., silicon (Si)), the losses created by the matching networks often becomes large enough to mitigate their usefulness.
FIG. 1 shows a conventional LNA 100. The conventional LNA 100 includes an input matching network 110, a first amplifier stage 120, a first inter-stage matching network 130, a second amplifier stage 140, a second inter-stage matching network 150, a third amplifier stage 160, and an output matching network 170. As is well known in the art, the matching networks 110, 130, 150 and 170 allow amplifier stages of different sizes and different gains to be used without experiencing significant signal loss through the LNA 100. The matching networks 110, 130, 150 and 170 also operate to reduce the overall noise figure of the LNA 100. However, the matching networks 110, 130, 150 and 170 also introduce losses into the LNA 100, as would any electrical component.
Thus, there is presently a need for an LNA which is compact (i.e., does not include matching networks) but which also has a minimum overall noise figure.